(1) Field of the Invention
This invention relates to roof bolts and, more particularly, to a roof bolt which is positioned in a bore hole drilled in a rock formation in a mine roof and which is held in place within the bore hole by both a mechanical anchor and by a quick-setting resin system.
(2) Description of the Prior Art
It is a well established practice in underground mining work, such as coal mining, tunnel excavation or the like, to reinforce or support the roof of the mine to prevent rock falls or cave-ins. The most common means presently used to support a mine roof is an elongated bolt or bar which is inserted into the rock formation above the mine roof in a bore hole and which is securely fixed in the bore hole by an anchoring means such as a mechanical anchor, a quick-setting resin which surrounds the end of the bolt within the hole, or both. The roof bolt, placed under tension, is used to hold a metal support plate in close engagement with the roof.
The mechanical anchor type of roof bolt is well known and has been used for many years in supporting mine roofs. Such roof bolts typically include an elongated bolt which has a head on one end and is threaded on the opposite end. A radially expanding gripping member, referred to as an expansion shell or gripper, and an internally threaded tapered nut or spreader are placed onto the threaded end of the bolt and the downward movement of the gripper is limited by a stop mechanism such as a nut or the like. The threaded end of the bolt, along with the gripper and spreader, is placed within the bore hole drilled in the rock formation until the gripping surface on the exterior of the gripper makes contact with the rock formation. The bolt is then rotated and because the gripper is constrained from rotating, the spreader is gradually drawn downward into the gripper to cause radial expansion thereof into tightly engaged contact with the wall of the bore hole.
The use of a mechanical anchor type of roof bolt has several disadvantages. Firstly, the strength of such a roof bolt is limited due to the nature of the anchorage and will typically only hold a tension of about 12,000-16,000 lbs. In addition, it is known that the holding power of the mechanical anchor releases over time due to creep, deterioration of the rock formation surrounding the expanded gripper, and the like. This causes the gripper to slip and the tension on the bolt decreases, thereby reducing the roof support.
A more recent and generally more acceptable development has been the use of a quick-setting resin type of bolting system which permits the roof bolt to be tensioned. The use of the term "resin" is meant to include any of the resin systems, adhesive systems, cementitious systems, grouting systems and the like which are known and used in the art. Anchor bolt assemblies relying solely on a resin to mount the roof bolt within a bore hole generally include a length of reinforcing rod, also known as rebar, and an elongated bolt threadedly joined together by a two position coupling. A capsule or series of capsules containing a quick-setting resin system, such as a polyester resin and a catalyst hardener, is positioned at the blind end of the drill hole and the anchor bolt assembly is inserted into the bore hole with the rebar end adjacent the resin capsules. The anchor bolt is then further inserted and rotated so as to rupture the capsules and mix together the resin and catalyst within the bore hole. The resin system components are mixed by the knurled or textured outer surface of the rebar and the mixture quickly sets and securely bonds the rebar to the rock formation. A stop means is provided in the coupling and limits axial advancement of the bolt into the coupling and ensures that initially the entire anchor bolt rotates. After the resin has cured, further turning of the bolt releases or breaks the stop mechanism in the coupling and permits the bolt alone to be rotated and to move upwardly within the bore hole while the remainder of the anchor bolt remains rigid. Sufficient torque can be applied to tension the bolt within the bore hole. Examples of such two position anchor bolt assemblies are shown in U.S. Pat. Nos. 3,877,235; 3,896,627; 4,023,373; 4,051,683; 4,122,681; 4,132,080; 4,192,631; 4,193,715; and 4,477,209.
These resin based anchor bolt systems are much stronger than conventional mechanical anchor bolts. The resin penetrates into the surrounding rock formation to unite the rock strata and to firmly hold the bolt in position in the bore hole. The resin also fills the space between the rock formation and the bolt along a substantial portion of its length. Such a bolt starts to fail at the yield strength of the elongated bolt rod and is typically torqued to a tension of up to about one-half the yield strength.
However, these anchor bolt systems have several disadvantages. The use of a processed rebar to make contact with the resin and the use of a coupling between the rebar and bolt results in a device which is much more expensive than conventional mechanical type roof bolts. Furthermore, an additional time factor is added to the installation of such roof bolts since an operator must wait until the resin is solidly cured before the bolt can be tensioned within the bore hole.
It is also known in the art to combine a mechanical type of anchor bolt with a quick-setting resin. See, for example, the systems shown in U.S. Pat. Nos. 4,160,614; 4,162,133; 4,194,858; and 4,299,515. In these systems, the bolt is rotated in a first direction to mix the resin components and is then rotated in a second direction to draw the spreader downward and expand the gripper. A means is provided to prevent expansion of the gripper while the bolt is being rotated in the first direction. The use of a system requiring two different directions of rotation causes a discontinuity in installing a roof bolt and has an obvious disadvantage. U.S. Pat. No. 4,419,805 discloses a combination mechanical anchor and resin bolt system which permits the resin components to be mixed and the anchor to be expanded and set by a continuous rotation of the bolt in one direction. However, this system requires the use of a breakable stop device which prevents axial movement of the spreader until after the resin has begun to cure. In addition, all of these anchor bolt systems have the disadvantage that the resin components are mixed together only through the rotation of the bolt and the mechanical connector. Such a mixing action is not as thorough or complete as is desired. Furthermore, none of these systems provides a means for continuously urging the resin upwards to completely fill the space between the rock formation and the bolt. The washers provided on some of these systems (such as element 90 in U.S. Pat. No. 4,419,805, element 29 in U.S. Pat. No. 4,162,133, and element 62 in U.S. Pat. No. 4,299,515) are merely stops which keep the resin from running down along the length of the bolt from the force of gravity.
Accordingly, it is an object of the present invention to provide a method and apparatus for anchoring roof bolts which combines the features of mechanical anchors and resin bonding but also provides positive and complete mixing of the resin components by an additional mixing mechanism. It is an object to do this with a roof bolt which is reasonably inexpensive and easy to manufacture; which also forces the resin upwards along the bolt during the mixing operation; which more violently mixes the resin, resulting in a shorter mix time; and, which eliminates the use of a two position coupling or delay mechanism.